Figure 15.

Comparison of neurogenesis in major arthropod lineages and Onychophora. Schematic sagittal sections through single hemi-neuromeres. For simplicity, only three CISs/NBs are depicted. Different colours (blue, yellow, pink, green) have been used to highlight well-characterized NP types. Insufficiently characterized NP types are depicted in light grey (indicating not necessarily homology across taxa). (A) Pycnogonida (predominantly based on Pseudopallene sp.). Few CISs form within the VNE. Unordered, tangential cell divisions occur in the VNE. Single GCs/INPs detach basally, forming a loose layer, in which first neurons start to differentiate. Apically, a central invagination forms, deepening with ongoing development. Large spindle-shaped NSCs differentiate and divide in tangential or slightly oblique orientation. Their smaller daughter cells immigrate independently. Sub-apical INPs divide in the anterior and posterior portions of the hemi-ganglion anlage. Prospective epidermis cells cover the invagination’s rim and later overgrow it. (B) Euchelicerata (predominantly based on Cupiennius salei). CISs form sequentially in the VNE. Unordered, mostly tangential cell divisions occur in the VNE. Mostly immature GCs detach and differentiate basally to the CISs. Several CISs transform into cell-rich units, being enclosed by glial-like sheath cells (representativeness for euchelicerates unclear). Scattered symmetrically dividing INPs are found close to the forming neuropil. The epidermis overgrows the hemi-neuromeres apically. (C) Myriapoda (predominantly based on Glomeris marginata). Similar to euchelicerates, but VNE cells are more closely packed. Spatial correlation of cell divisions with CISs is observable, but existence of a specialized NP remains unresolved. In advanced stages, a central invagination forms, being later overgrown by the epidermis. Spindle-shaped NPs and sub-apical NPs occur, but cell types and division patterns await reinvestigation. (D) ‘Crustacea’ (predominantly based on Orchestia cavimana). NBs form sequentially in a stereotyped division sequence. They are maintained apically and produce GMCs by asymmetrical radial divisions. Some NBs intermittently switch to symmetrical tangential divisions to generate epidermal cells. GMCs divide (typically) once to produce immature neurons and/or glial cells. NB fate remains unresolved. (E) Hexapoda (predominantly based on Drosophila melanogaster and Schistocerca sp.). Similar to crustaceans, but sequentially specified NBs immigrate into sub-apical position prior to proliferation. Most NBs undergo apoptosis after generation of their lineage. Association of NBs with sheath and cap cells has been reported in grasshoppers only, their plesiomorphic character being therefore questionable and here omitted. (F) Onychophora (predominantly based on Euperipatoides kanangrensis and E. rowelli). NPs immigrate individually from the VNE without recognizable spatio-temporal pattern. Immigrated NPs form a sub-apical layer and divide at least once in symmetrical fashion, producing immature neurons (potentially also glial cells) and presumably some INPs. First neurons differentiate basal-most and extend growth cones anteriorly. In advanced stages, segmental ‘ventral organs’ develop as transformations of the VNE.